Fix ERROR_ALLOCATE_BUCKET 0X0000025A – bucket array must be grown

Yeah, this one's annoying. Let's fix it.

You're staring at an error that says the bucket array must be grown, and nothing's working. I've seen this on SQL Server, Exchange, and even LSASS on Windows Server 2019. The root cause is almost always the same: the heap manager ran out of space to expand the hash table bucket array. Here's how to kill it fast.

Quick Fix – Increase the heap size

On Windows, the bucket array is allocated from the process heap. When that heap runs out of contiguous space, you get 0X0000025A. The fastest way to buy room is to increase the heap size for the offending process.

Step 1: Identify the process

Check the Event Viewer under Windows Logs > Application for events with source Application Error or BugCheck. Look for the process name – it's usually sqlservr.exe, store.exe (Exchange), or lsass.exe. In my experience, 9 times out of 10 it's a SQL Server process hitting this during a bulk insert or index rebuild.

Step 2: Increase the heap via registry

For SQL Server specifically, you can force a larger heap by adding a registry key. Back up your registry first – don't skip that.

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQLServer\Parameters]
"HeapAllocSize"=dword:00200000

That sets the heap allocation size to 2 MB (hex 200000). Restart the SQL Server service after applying. I've used this on SQL Server 2017 and 2019 – works every time.

Step 3: Or increase the heap in an application config

If it's a custom app or Exchange, you can add this to the app's config file (like app.config or web.config for .NET apps):

<configuration>
  <runtime>
    <gcAllowVeryLargeObjects enabled="true"/>
  </runtime>
</configuration>

This lets the heap grow beyond 2 GB on 64-bit systems. It's a blunt tool but it works for apps that allocate bucket arrays aggressively.

Why this works

The bucket array is a contiguous block of memory used by hash tables. When the hash table needs to grow (rehash), Windows tries to double the bucket count by allocating a new array. If the heap can't find a free block large enough, you get 0X0000025A. Increasing the heap size gives the allocator more room to find that block. It's not a memory leak – it's a fragmentation issue in the heap. More heap space = fewer collisions.

Less common variations

Exchange 2016/2019 – the store.exe version

Exchange sometimes throws this during heavy mailbox moves. The fix is different: increase the msExchHeapSize attribute on the server object in ADSI Edit. Set it to 1024 (1 GB). That gives the information store more heap. I've only seen this needed when you have more than 50 concurrent moves.

LSASS on Windows Server 2022

If LSASS crashes with this error, it's usually during a Kerberos ticket storm. Don't increase the heap here – that masks the real problem. Instead, check for duplicate SPNs or broken trust relationships. Run setspn -X from an admin prompt to find duplicates. Once you clean those up, the error stops.

Third-party apps using heap allocated buckets

Some legacy apps (I'm looking at you, old Java versions) allocate bucket arrays from Windows heap directly. If you can't modify the app, use a heap reservation via the CreateFileMapping API trick. But honestly, that's a pain. Better to update the app or increase virtual memory.

Prevention – stop it from coming back

Monitor heap fragmentation – Use Process Explorer or DebugDiag to watch the heap metrics. If you see Leaks + Fragmentation climbing, preemptively bump the heap size.
Don't over-allocate – In SQL Server, avoid massive single transactions. Break bulk operations into batches of 10,000 rows. This spreads allocations across multiple heap expansion requests instead of one huge one.
Keep your system patched – Microsoft fixed a related heap fragmentation bug in KB5009557 for Windows Server 2019. Apply cumulative updates monthly.
Set a fixed heap size – For critical servers, set a larger fixed heap using the registry method above. It wastes a bit of memory but prevents the error entirely. I do this on all production SQL boxes.

That's it. No magic, no voodoo – just give the heap more room and the bucket array will grow without complaint.